1. Field of the Invention
This invention relates to a high temperature superconductor. More specifically, it relates to a superconductor usable at the temperature of liquid nitrogen or a higher temperature, and a superconductor usable at the temperature of liquid nitrogen or a higher temperature, being low in anisotropy, and having a high critical current density, and a process for their preparation.
2. Description of the Prior Art
Known copper oxide superconductors having superconducting transition temperatures, Tc's, of 110 K or more are Tl and Hg type copper oxide superconductors. These copper oxide superconductors have high Tc values, but they contain harmful and rare elements such as Tl or Hg. Thus, they posed marked problems, such as the necessity for anti-pollution measures, the need for cost reduction, and difficulty in securing such rare resources.
With those conventional superconductors, let the coherence length in the intrafacial direction of the CuO.sub.2 plane (ab plane) in its crystal structure be .xi.ab, the coherence length in a direction perpendicular to the CuO.sub.2 plane (c-axis direction) be .xi.c, and the anisotropy .gamma. of the superconducting properties be expressed as .xi.ab/.xi.c. Then, .gamma.=5 for the composition YBa.sub.2 Cu.sub.3 O.sub.7-y with the lowest anisotropy, and this value was considerably high. Thus, those conventional oxide superconductors have had short coherence lengths in the c-axis direction perpendicular to the CuO.sub.2 plane, which, in turn, have made critical current densities under high temperature, high magnetic field conditions small. These facts have been an obstacle to their practical use.